Book Description

Due to their efficient heat and mass transfer potential, impinging jets have received attention in various applications. Heat transfer and flow characteristics of rectangular turbulent impinging jets issued from a 24:1 aspect ratio and 24:1 contraction ratio nozzle were investigated experimentally and numerically. In the heat transfer measurements; a thin stainlessƯ-steel foil was utilized to obtain isoƯflux boundary conditions on the impingement surface. The target plate was free to translate in the lateral direction and the heat transfer distributions were determined at 0d"/Wd"0 with the microƯ-thermocouples placed underneath the foil. The measurements were conducted for Re=8900-48600 at nozzleƯ-to-Ưtarget spacing of 0.5d"/Wd"2.0. Both semi and fully confined jets were investigated. Heat transfer coefficients at Re=28100, 36800, 45600 and H/W=4.0 were determined by using adiabaticƯ-wall temperatures and the distributions were compared with those of the wall shear stress. Off-center peaks were observed at high Re and low H/W. Since the wall distributions are susceptible to nozzleƯ-exit conditions, velocity and turbulence profiles at the nozzleƯ-exit were measured for the velocity range of interest. Additionally, near-Ưwall mean velocity and turbulence profiles were determined at Re=21500 and 36800 at H/W = 4.0 to have a better understanding of the secondary peaks in the wall distributions. Numerical computations were performed by using several different turbulence models (k-#, k-#, V2F and Reynolds stress models). In wallƯ-bounded turbulent flows, near-Ưwall modeling is crucial. Therefore, the turbulence models eliminating wall functions such as the k-# and V2F models may be superior for modeling impingement flows. The numerical results showed reasonable agreement with the experimental data for local heat transfer and skin friction coefficient distributions. The occurrence of the secondary peaks was predicted by the k-# and V2F models, and for a few cases with the low-Re-k-# models. Near-Ưwall measurements along with the computed profiles were used to describe the s̀̀econdary peak'' phenomena. It was shown that the increase in turbulence production in the wallƯ-streamwise direction enhances turbulent momentum and heat transport in the wall-Ưnormal direction which lead to secondary peaks in the wall distributions. The possibility of improving surface heat transfer with fully-Ưdeveloped jets was also explored numerically as a case study.